There is currently an increasing problem in many countries with the smuggling of illicit or illegal substances, such as drugs and narcotics, and also various substances which are legal but subject to high tariffs making smuggling attractive; generally the illicit or illegal substances are the major concern. A further problem is the transportation of explosives, either illegally in the nature of smuggling, or with the intention of being used as part of a terrorist threat or attack on a ship, aircraft or the like.
Consequently, there is increasing demand for detection equipment for use at airports, seaports, border crossings, etc. to enable authorities of individual countries to detect such substances, whether carried by individuals, in baggage carried by an individual, or in large, commercial transportation containers and the like.
Such detection equipment is used increasingly in the screening of baggage for explosives, and commonly relies on the collection of vapors which are subsequently analyzed by mass spectrometric or chromatographic techniques. Detection of modern plastic explosives and illicit drugs, such as heroin and cocaine, by such vapor collection is difficult, and frequently impossible, because of factors such as: the extremely low vapor pressures of explosives and narcotics; the small amounts of vapors emanating from these explosives and drugs requiring high volume sampling and very high sensitivity of detection; these explosives and drugs being easily concealed in a variety of baggage or personal articles inhibiting the collection of vapors; the frequent presence of toilet articles, perfumes, cosmetics and the like on persons, belongings, and baggage, producing vapors containing molecules with some properties similar to the targets of interest, thus further complicating the analysis.
Handling of explosives and/or drugs etc. and attempts at their concealment results in minute surface contamination from trace residues of these substances. Similarly, the packaging of drugs, such as cocaine and heroin, for concealment in baggage or cargo is equally difficult to achieve without similar surface contamination. With exceptional measures being taken, such traces will be present, and are in sufficient quantity, although minute, to enable them to be collected through a variety of means, and to be subsequently analyzed by IMS.
Inherent in the detection of small quantities is the possibility of unacceptable False Alarm Rates (FAR).
In general, the smaller the quantity that can be detected, i.e. the more sensitive the detection equipment, the greater the possibility of a False Alarm. This can be caused by detection of spurious trace quantities present from another source, e.g. because the user has just handled a contaminated article; residue from a previous test that was lodged in the apparatus, but becomes dislodged and drawn through the apparatus.
Thus, IMS affords a well-established technique for the detection of drugs and explosives, but because of its high sensitivity interferences can occur and cause false alarms. If FAR are excessive, the instrument can have little practical value to security screeners and customs officers, and users will have little confidence in the instrument. It would enhance the value of the equipment if the FAR is reduced to the minimum possible.
A major cause of interference and instability in IMS is the presence of water vapor and other contaminants in drift gas. For obvious operational convenience, air is generally used as the drift gas in customs and security screening locations. Ambient air has to be dried to a high degree before use, to avoid these problems. Additionally, chemical scrubbing with charcoal or the like is used to remove other contaminants from the air, such as trace hydrocarbons often present in urban atmospheres from automotive traffic, airport traffic, or industrial operations. In laboratory situations IMS instrumentation can be run from compressed, dried air, called zero air, but this is impractical and too costly for actual use.
Another source of interference is the presence of cosmetics, toilet articles and other articles on travellers and in their belongings. These may cause spurious alarms due to chemical signature interferences in IMS, unless the IMS incorporates superior peak discrimination capabilities. Some legal substances can give chemical signatures that are difficult to differentiate from illegal substances of interest.